What Caused The Loudest Noise Ever Heard Underwater In 1997?

For any wannabe Ernest Shackleton, it might seem that the 21st century doesn't provide many opportunities left for real exploration, but don't despair as there is still one place on Earth full of unanswered mysteries.

In 1997, the loudest underwater sound ever recorded was detected by hydrophones set 5,000 miles apart in the Pacific Ocean.

Initially built to assist in the detection of Soviet submarines, the listening system had never heard anything like the ultra-low frequency sound emanating from a point off the southern coast of Chile, that lasted for one minute and never to be heard again.

And for the team of ten oceanographers at NOAA, lead by Dr Christopher Fox, who were left to piece the puzzle together, it was "baffling" and had no immediate scientific explanation.

Now twenty years later, the source of the sound, which has come to be known colloquially as the bloop, (as well as whistle, slowdown, upsweep and even Gregorian chant), is still a mystery.

Over the years, there were many possible theories put forward.

When it first happen, Dr Christopher Fox went to the United States Navy intelligence, to ask if it could have been a manmade sound from a bomb detonating or a submarine, but they said it wasn't them.

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Then in 2002 reports said it was consistent with large marine animals - perhaps a many-tentacled giant squid (the largest on record at the time was 60ft long).

Then Fox and his team explored the possibility of it being a blue whale, but if so it would have been more powerful than any previous call recorded on Earth.

The bloop was also explained as being consistent with noises generated by icequakes in large icebergs, or large icebergs scraping the ocean floor

By 2012 the NOAA concluded that the noise was an icequake, and scientist Robert Dziak told Wired: "Each year there are tens of thousands of what we call 'icequakes' created by the cracking and melting of sea ice and ice calving off glaciers into the ocean.

Today the most likely explanation is that it was ice related, but this hasn't stopped the most bizarre explanation of all - that it is a fictional creature from H.P Lovecraft's novel 'The Call Of Cthulhu'.

As the roughly triangulated origin of the bloop is approximately 950 nautical miles (1,760 km) from the precisely-described location of R'lyen a sunken extra-dimensional city.

In the documentary, Dr Fox says he is happy no conclusive answer has been found: "I am glad there are still mysteries on earth and in the universe."